Wide capture range reference-less frequency detector

ABSTRACT

A reference-less frequency detector circuit includes a sampling circuit that is configured to generate a frequency control voltage and a switch circuit control signal based on a frequency difference between a clock signal frequency and an input data rate. The frequency control voltage has a frequency down indication and a frequency up indication. A voltage-to-current converter circuit is coupled to the sampling circuit and is configured to convert the frequency control voltage to a frequency control current based on the switch circuit control signal. The voltage-to-current converter circuit includes an output switch circuit controlled by the switch control signal and is configured to have substantially equal respective latencies for the frequency down indication and the frequency up indication.

FIELD OF THE INVENTION

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application Ser. No. 62/436,200, filed onDec. 19, 2016, which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present disclosure is related to frequency detectors and, inparticular, to reference-less frequency detectors.

BACKGROUND

A conventional clock/data recovery (CDR) uses a reference clock in orderto bring the voltage controlled oscillator (VCO) output frequency intothe vicinity of the data rate before phase acquisition. In certainapplications, such as power and/or area limited circuits, a referenceclock may not be present or may be difficult to route to the CDR.Accordingly, reference-less CDRs may be used in such situations.

However, in frequency detector (FD) of reference-less CDRs, the latencyof frequency down control signals is much larger than the latency offrequency up control signals due to asymmetrical architecture within thefrequency detector. Thus, when the input data rate is much lower thanthe VCO frequency, the frequency detector cannot reduce the VCO'sfrequency low enough to match the input data rate. Thus, the frequencycannot lock to the data rate and the data may be lost.

SUMMARY

Various examples are now described to introduce a selection of conceptsin a simplified form that are further described below in the detaileddescription. The summary is not intended to identify key or essentialfeatures of the claimed subject matter, nor is it intended to be used tolimit the scope of the claimed subject matter.

A reference-less frequency detector circuit including a sampling circuitconfigured to generate a frequency control voltage and a switch circuitcontrol signal based on a frequency difference between a clock signalfrequency and an input data rate, the frequency control voltagecomprising a frequency down indication and a frequency up indication. Avoltage-to-current converter circuit is coupled to the sampling circuitand configured to convert the frequency control voltage to a frequencycontrol current based on the switch circuit control signal, thevoltage-to-current converter circuit comprising an output switch circuitcontrolled by the switch control signal and configured to havesubstantially equal respective latencies for the frequency downindication and the frequency up indication.

A clock/data recovery system includes a voltage controlled oscillatorconfigured to generate a clock signal. A phase detector is coupled tothe voltage controlled oscillator and configured to generate a buffereddata signal, and a phase difference control signal based on an inputdata signal and the clock signal, wherein the phase difference controlsignal is indicative of a phase difference between the clock signal andthe input data signal. A reference-less frequency detector coupled tothe voltage controlled oscillator and configured to generate a frequencydifference control signal indicative of a frequency difference betweenthe input data signal and the clock signal. The frequency detectorincludes sampling circuit configured to generate a frequency controlvoltage and a switch circuit control signal based on the frequencydifference between the clock signal and the input data signal, thefrequency control voltage comprising a frequency down indication and afrequency up indication. A voltage-to-current converter circuit iscoupled to the sampling circuit and configured to convert the frequencycontrol voltage to a frequency control current in response to the switchcircuit control signal, the voltage-to-current converter circuitcomprising an output switch circuit controlled by the switch controlsignal and configured to have substantially equal respective latenciesfor the frequency down indication and the frequency up indication. Afilter is coupled to the phase detector and the frequency detector andconfigured to output a control signal to the voltage controlledoscillator, wherein the control signal provides an indication of a phasedifference and a frequency difference between the input data signal andthe clock signal.

A method for generating a frequency control signal from a reference-lessfrequency detector includes generating an up/down control voltage,wherein the up/down control voltage is indicative of a frequencydifference between an input data rate and a clock frequency andcomprises an up frequency indication voltage and a down frequencyindication voltage. An on/off control signal is generated based on thedata rate and the clock frequency. The frequency control current isgenerated based on the up and down control voltages and the on/offcontrol signal, wherein the on/off control signal controls an outputswitch circuit of a voltage-to-current converter circuit such that alatency of the up frequency indication voltage is equal to a latency ofthe down frequency indication voltage through the output switch circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a reference-less frequencydetector, according to various embodiments.

FIG. 2 illustrates a schematic diagram of a voltage-to-current convertercircuit, according to the embodiment of FIG. 1.

FIGS. 3A, 3B illustrate plots of operational signals of thereference-less frequency detector, according to the embodiment of FIG.1.

FIG. 4 illustrates a block diagram of a clock/data recovery systemincluding a reference-less frequency detector, according to variousembodiments.

FIG. 5 illustrates a flowchart of a method for operation of thereference-less frequency detector, according to various embodiments.

DETAILED DESCRIPTION

Some of the challenges noted above, as well as others, can be addressedby implementing the reference-less frequency detector having an outputswitch circuit. The output switch circuit includes a symmetricalarchitecture that provides equal respective latencies for both afrequency down indication as well as a frequency up indication for a VCOcircuit. The VCO control signal latency is also reduced as compared toconventional frequency detector circuits.

FIG. 1 illustrates a reference-less frequency detector 150, according tovarious embodiments. The frequency detector 150 includes a samplingcircuit 100 coupled to a voltage-to-current converter circuit 105.

The sampling circuit 100 includes flip-flops 101, 102, 103 (e.g., Dflip-flops). A data signal D_(IN) and a clock signal CLK are input tothe sampling circuit 100. The sampling circuit generates a frequencycontrol signal and a switch circuit control signal based on a frequencydifference between the clock signal CLK and data signal D_(IN).

In an embodiment, the sampling circuit 100 shows a delay cell 108 whosedelay time is ¼ of a bit time. However, the delay cell 108 is used forillustrating that D_(IN) has a delayed version, D_(IN2). In other words,the D_(IN2) signal is a delayed version of the D_(IN) signal. TheD_(IN2) signal is coupled to the clock input of one flip-flop 102 whilethe D_(IN) signal is coupled to the clock input of another flip-flop101.

The CLK signal may be input from a voltage controlled oscillator (VCO),as described subsequently. Other embodiments may input the clock signalCLK from other clock generation circuits.

The D_(IN) signal and the CLK signal are provided as the clock input andthe data input, respectively, of D flip-flop 101. D_(IN2) and the CLKsignal are provided as the clock input and the data input, respectively,of D flip-flop 102.

D flip-flop 102 samples the CLK signal at the rising edges of theD_(IN2) signal to generate an output signal Q2. D flip-flop 101 samplesthe CLK signal at the rising edges of the D_(IN) signal to generate anoutput signal Q1.

The period of signals Q1 and Q2 are substantially equal and proportional(e.g., inversely proportional) to the frequency difference between theD_(IN) signal and CLK signal. Signals Q1 and Q2 contain informationregarding the frequency difference between the D_(IN) signal and CLKsignal. Specifically, when signal Q2 leads signal Q1, the CLK signal hasa higher frequency than the D_(IN) signal. When signal Q2 lags signalQ1, the CLK signal has a lower frequency than the D_(IN) signal.

Signal Q3 indicates which of signals Q1 and Q2 leads the other. Inparticular, when signal Q2 leads signal Q1, signal Q3 is a logic lowindicating that the frequency of the CLK signal is higher than thefrequency of the D_(IN) signal. The logic low indicates to the VCO thatit needs to decrease its clock frequency to match the data frequency(e.g., frequency down indication). On the other hand, when signal Q2lags signal Q1, signal Q3 is a logic high indicating that the frequencyof the CLK signal is lower than the frequency of the D_(IN) signal. Thelogic high indicates to the VCO that it needs to increase its clockfrequency to match the data frequency (e.g., frequency up indication).Thus, signal Q3 may be referred to as an up or down control voltage,depending on the signal state, or frequency control signal since, whenconverted to a frequency control current (e.g., an up/down currentsignal) by the voltage-to-current converter circuit 105, it may be usedby the voltage controlled oscillator to know when to increase itsfrequency (i.e., up) or decrease its frequency (e.g., down) tocorrespond to the frequency of the input data.

Signal Q2 may be referred to as an on/off signal or switch circuitcontrol signal. This signal is used to turn on or off an output switchcircuit in the voltage-to-current converter circuit 105, as describedsubsequently with reference to FIG. 2.

Signals Q₂ and Q₃ are used in the voltage-to-current converter circuit105 so inverters 130, 131 are used to generate these signals from theirrespective complementary signals Q2 and Q3. Voltage-to-current convertercircuit 105 is coupled to the output of the D flip-flop 103, the outputof the D flip-flop 102, the output of inverter 130, and the output ofinverter 131. In an embodiment, the output of the voltage-to-currentconverter circuit 105 (e.g., up/down current signal) is input to afilter circuit, as described subsequently with reference to FIG. 4.

FIG. 2 illustrates a schematic diagram of a voltage-to-current convertercircuit 105, according to the embodiment of FIG. 1. The schematicdiagram shows one implementation of the voltage-to-current convertercircuit 105. This circuit 105 includes the output switch circuit 200.

The circuit 105 includes two series circuits 280, 281. Each seriescircuit 280, 281 includes respective first and second transistors 222,223, 224, 225 coupled in series (e.g., drain-to-drain) with each otherand being of complementary type with each other. For example, transistor222 may be a p-type transistor (e.g., PFET) while transistor 223 may bean n-type transistor (e.g., NFET). Similarly, transistor 224 may be ap-type transistor (e.g., PFET) while transistor 225 may be an n-typetransistor (e.g., NFET). The two series circuits 280, 281 are coupledtogether in parallel at first and second common nodes 290, 291 such thattransistors 222, 224 have their sources coupled to the first common node290 and transistors 223, 225 have their sources coupled to the secondcommon node 291.

The first common node 290 is coupled to a supply voltage node that maybe powered to V_(DD) during circuit operation. The second common node291 is coupled to a current source 230 that is also coupled to a circuitreference voltage node (e.g., circuit ground).

Transistor 222 and transistor 224 are coupled in a diode configuration.In other words, their control gates are coupled to their respectivedrain nodes. Thus, these transistors 222, 224 provide a diode voltagedrop in the circuit during circuit operation.

The control gate of transistor 223 is coupled to and controlled bysignal Q₃ such that the transistor 223 turns on and conducts when Q₃ islow. The control gate of transistor 225 is coupled to and controlled bysignal Q₃ such that the transistor 225 turns on and conducts when Q₃ ishigh.

Transistors 220 and 221 are coupled in series (e.g., drain-to-drain)between the supply voltage node and the circuit reference voltage node.The source of transistor 220 is coupled to the supply voltage node andthe source of transistor 221 is coupled to the circuit reference voltagenode. The control gate of transistor 220 is coupled to the control gateof transistor 222. The control gate of transistor 221 is coupled to acontrol gate of transistor 206 in the output switch circuit 200.Transistor 221 is also coupled in a diode configuration (e.g., controlgate coupled to drain) so that it provides a diode voltage drop duringcircuit operation.

The output switch circuit 200 includes two series circuits 282, 283.Each series circuit 282, 283 includes respective first and secondtransistors 202, 203, 204, 205 coupled in series (e.g., drain-to-drain)with each other and being of complementary type with each other. Forexample, transistor 202 may be a p-type field effect transistor (e.g.,PFET) while transistor 203 may be an n-type transistor (e.g., NFET).Similarly, transistor 204 may be a p-type transistor (e.g., PFET) whiletransistor 205 may be an n-type transistor (e.g., NFET). The two seriescircuits 282, 283 are coupled together in parallel at first and secondcommon nodes 292, 293 such that transistors 202, 204 have their sourcescoupled to the first common node 292 and transistors 203, 205 have theirsources coupled to the second common node 293.

The first common node 292 is coupled the drain of transistor 201 whosesource is coupled to the supply voltage node that may be powered toV_(DD) during circuit operation. The second common node 293 is coupledto the drain of transistor 206 whose source is coupled to the circuitreference voltage node (e.g., circuit ground). The control gate oftransistor 201 is coupled to the control gate of transistor 224. Thecontrol gate of transistor 206 is coupled to the control gate oftransistor 221.

Signal Q2 is coupled to the control gates of transistors 202, 205.Signal Q₂ is coupled to the control gates of transistors 203, 204. Acommon node between transistor 202 and 203 is the output of thevoltage-to-current converter circuit 105. A common node betweentransistor 204 and transistor 205 is coupled to a voltage dividercircuit formed by resistances R1 210 and R2 211. The resistance R1 210is also coupled to the power supply node while the resistance R2 211 isalso coupled to the circuit reference voltage node (e.g., circuitground).

In operation, the voltage-to-current converter circuit has two modes ofoperation. When the clock frequency is less than the data frequency, thecircuit operates in a frequency up mode to cause the VCO frequency toincrease. When the clock frequency is greater than the data frequency,the circuit 105 operates in a frequency down mode to cause the VCOfrequency to decrease.

In the frequency up mode, when the input data rate is higher than theVCO frequency, frequency control voltage signal Q3 is at logical “1”.Transistors 224, 225, 201 are ON while transistors 206, 220, 221, 222,223 are OFF. When switch circuit control signal Q2 goes low, transistor202 is ON while transistor 204 is OFF. The current signal output fromthe voltage-to-current converter circuit 105 (i.e., conversion oflogical high Q3 voltage) will charge the following filter (e.g., loopfilter) and the VCO frequency increases.

The total latency of signal Q2 through the voltage-to-current convertercircuit 105 is only the delay of transistor 202. This means thefrequency capture range in the frequency up mode is approximately fourtimes of that in prior art. When signal Q2 goes high, transistor 202 isOFF while transistor 204 is ON. Through transistor 204, the drainvoltage of transistor 201 is at a bias voltage approximately half of thesupply voltage (e.g., V_(DD)/2). This bias voltage is generated with thetwo series connected resistances R1 210 and R2 211, between the powersupply node at V_(DD) and the circuit reference voltage node at circuitground potential. Keeping the second series circuit at V_(DD)/2 keepstransistor 201 on when the voltage-to-current converter circuit turnsoff and reduces the time required to eventually go to the expectedvoltage at the output when the voltage-to-current converter circuitturns on. This can help to further reduce the latency of signal Q2 byreducing the setup time of current in transistor 201.

In the frequency down mode, when the input data rate is lower than theVCO frequency, frequency control voltage signal Q3 is at logical “0”.Transistors 201, 224, 225 are OFF while transistors 206, 220, 221, 222,223, are ON. When switch circuit control signal Q2 goes low, transistor203 is ON while transistor 205 is OFF. The current signal output of thevoltage-to-current converter circuit 105 will discharge the followingfilter. Thus, the VCO frequency will decrease to match the slower datarate.

The total latency of Q2 is only the delay of transistor 203. This meansthe frequency capture range in the frequency down mode is approximatelysix times of that in prior art. When Q2 goes high, transistor 203 is OFFwhile transistor 205 is ON. Through transistor 205, the drain voltage oftransistor 206 is at a bias voltage that is approximately half thesupply voltage (e.g., V_(DD)/2). The bias voltage is generated by thetwo series connected resistances R1 210 and R2 211, between the powersupply voltage and the circuit ground reference voltage, acting at avoltage divider circuit. This can help to further reduce the latency ofQ2 by reducing the setup time of current in transistor 206.

The latency of signal Q2 determines the minimum allowed pulse width ofQ2 as well as the maximum allowed difference between the input data rateand the VCO frequency (i.e., the frequency capture range). It can beseen that in both the frequency up mode and the frequency down mode, thelatency of Q2 is the same. In both cases the latency is one transistordelay. This is unlike the prior art where the latency for Q2 is not onlygreater (e.g., the sum of multiple transistors) but it is notsymmetrical. Conventionally, the Q2 latency is greater for the frequencydown mode as compared to the frequency up mode. Thus, the conventionalfrequency capture ranges are substantially less than the disclosedembodiments as well as being different between the frequency up mode andthe frequency down mode.

FIGS. 3A, 3B illustrate plots of operational signals of thereference-less frequency detector, according to the embodiment ofFIG. 1. The plot of FIG. 3A shows the case where the data rate isgreater than the VCO frequency. In this case, the signal Q3 is at alogical high. The plot of FIG. 3B shows the case where the data rate isless than the VCO frequency. In this case, the signal Q3 is at a logicallow.

FIG. 4 illustrates a block diagram of a clock/data recovery systemincluding a reference-less frequency detector, according to variousembodiments. The clock/data recovery system may be used forsynchronizing an input data signal (D_(IN)) with a VCO 420 in order torecover the data transmitted via a noisy channel. This block diagram isfor purposes of illustrating only one possible implementation of thedisclosed embodiment of the reference-less frequency detector 150.

The system includes a phase detector 410 coupled to the input datasignal D_(IN). The phase detector 410 is also coupled to the clocksignal CLK generated by the VCO 420. The phase detector 410 compares thephase of the data in signal D_(IN) with the input clock signal CLK togenerate a phase difference signal PD-OUT that is based on the phasedifference between the input data signal D_(IN) and the VCO clock CLK.The PD-OUT signal is input to a loop filter 440.

The phase detector 410 also outputs a buffered data signal D_(INB) basedon the input data signal D_(IN). The input data signal D_(IN) is inputto the reference-less frequency detector 150 as described previouslywith reference to FIGS. 1 and 2. The VCO clock CLK is also input to thereference-less frequency detector 150 as described previously. Thereference-less frequency detector 150 generates the frequency differencesignal FD-OUT that is the up/down current signal based on the frequencydifference between the input data signal D_(IN) and the VCO clock CLK.

The loop filter 440 now has the phase difference signal and thefrequency difference signal as inputs. The loop filter 440 can nowgenerate the VCO control signal VCTRL that is input to the VCO 420 toadjust both the phase and the frequency of the VCO 420 to more closelyalign the clock signal CLK with the input data signal D_(IN) in bothphase and frequency.

The now phase and frequency aligned clock signal CLK is input to are-timer circuit 430 to clock out the buffered data signal D_(INB). TheD_(OUT) signal from the re-timer circuit 430 is the recovered datasignal.

FIG. 5 illustrates a flowchart of a method for operation of thereference-less frequency detector, according to various embodiments. Inblock 501, an up/down control voltage is generated. The up/down controlvoltage is indicative of a frequency difference between a data signaland a clock signal and comprises one of either an up frequencyindication voltage or a down frequency indication voltage.

In block 503, an on/off control signal is generated based on the datasignal and the clock signal. The on/off control signal comprises one ofeither an on voltage or an off voltage. In block 505, an up/down controlcurrent is generated based on the up/down control voltage and the on/offcontrol signal. The up/down control current comprises one of either anup control current or a down control current. The on/off control signalcontrols an output switch circuit of a voltage-to-current convertercircuit such that a voltage generation latency of the up frequencyindication voltage is equal to the voltage generation latency of thedown frequency indication voltage through the output switch circuit.

In one embodiment of a system for generating a frequency control signalfrom a reference-less frequency detector, the system includes voltagegeneration means for generating an up/down control voltage, wherein theup/down control voltage is indicative of a frequency difference betweenan input data rate and a clock frequency and comprises an up frequencyindication voltage and a down frequency indication voltage. The systemfurther includes control signal means for generating a control signalbased on the data rate and the clock frequency. In one embodiment, thecontrol signal is an on/off signal. The system further includes acurrent generation means for generating the frequency control currentbased on the up/down control voltage and the control signal, wherein thecontrol signal controls an output switch circuit of a voltage-to-currentconverter circuit such that a latency of the up frequency indicationvoltage is equal to a latency of the down frequency indication voltagethrough the output switch circuit.

One embodiment of the above system includes a voltage converter meansfor converting a voltage to a current, the voltage converter meansoperating in a frequency up mode based on the input data rate beinggreater than the clock signal frequency and, alternatively, in afrequency down mode based on the input data rate being less than theclock signal frequency.

Further aspects and embodiments of the invention are described in thefollowing clauses.

Clause 1. A reference-less frequency detector circuit comprising:

a sampling circuit configured to generate a frequency control voltageand a switch circuit control signal based on a frequency differencebetween a clock signal frequency and an input data rate, the frequencycontrol voltage comprising a frequency down indication and a frequencyup indication; and

a voltage-to-current converter circuit coupled to the sampling circuitand configured to convert the frequency control voltage to a frequencycontrol current based on the switch circuit control signal, thevoltage-to-current converter circuit comprising an output switch circuitcontrolled by the switch control signal and configured to havesubstantially equal respective latencies for the frequency downindication and the frequency up indication.

Clause 2. The circuit of clause 1, wherein the output switching circuitcomprises:

a first series circuit comprising first and second transistors coupledin series with each other and having a complementary type with eachother, a gate of the first transistor coupled to the output of thesampling circuit and a gate of the second transistor coupled to aninverse of the output of the sampling circuit, wherein an output nodefor the output switching circuit is a common node between the first andsecond transistors;

a second series circuit comprising third and fourth transistors coupledin series with each other and having a complementary type with eachother, a gate of the third transistor coupled to the inverse of theoutput of the sampling circuit and a gate of the fourth transistorcoupled to the output of the sampling circuit, wherein the first andsecond series circuits are coupled together in parallel at first andsecond common nodes;

a fifth transistor coupled between a supply node and the first commonnode:

a sixth transistor coupled between a circuit reference node and thesecond common node; and

a voltage divider network coupled between the supply node and thecircuit reference node, the voltage divider network configured toprovide a voltage to a common node between the third and fourthtransistors.

Clause 3. The circuit of any one of clauses 1-2, wherein the circuitreference node is circuit ground.

Clause 4. The circuit of any one of clauses 1-3, wherein the first andthird transistors are p-type field effect transistors (PFETs) and thesecond and fourth transistors are NFETs.

Clause 5. The circuit any one of clauses 1-4, wherein thevoltage-to-current converter circuit comprises:

a first pair of transistors coupled in series with each other and havinga complementary type with each other, wherein a gate of one of the firstpair of transistors is coupled to an output of a sampling circuit andconfigured to operate in response to a first state of the frequencycontrol voltage;

a second pair of transistors coupled in series with each other andhaving a complementary type with each other, wherein a gate of one ofthe second pair of transistors is coupled to the output of the samplingcircuit and configured to operate in response to a second state of thefrequency control voltage, the second state being an inverse of thefirst state, wherein the first and second pair of transistors arecoupled in parallel at a first common node and a second common node, thefirst common node coupled to the supply node; and

a current source coupled between the circuit reference node and thesecond common node.

Clause 6. The circuit of any one of clauses 1-5, wherein remainingtransistors of the first and second pairs of transistors are eachcoupled in a diode configuration and coupled to the supply node.

Clause 7. The circuit of any one of clauses 1-6, wherein a gate of thefifth transistor is coupled to a gate of the remaining transistor of thesecond pair of transistors.

Clause 8. The circuit of any one of clauses 1-7, wherein thesubstantially equal respective latencies for the frequency downindication and the frequency up indication are each based on a onetransistor latency.

Clause 9. The circuit of any one of clauses 1-8, wherein the onetransistor latency for the frequency up indication comprises the firsttransistor and the one transistor latency for the frequency downindication comprises the second transistor.

Clause 10. A clock/data recovery system comprising:

a voltage controlled oscillator configured to generate a clock signal;

a phase detector coupled to the voltage controlled oscillator andconfigured to generate a buffered data signal and a phase differencecontrol signal based on an input data signal and the clock signal,wherein the phase difference control signal is indicative of a phasedifference between the clock signal and the input data signal;

a reference-less frequency detector coupled to the voltage controlledoscillator and configured to generate a frequency difference controlsignal indicative of a frequency difference between the input datasignal and the clock signal, the frequency detector comprising:

a sampling circuit configured to generate a frequency control voltageand a switch circuit control signal based on the frequency differencebetween the clock signal and the input data signal, the frequencycontrol voltage comprising a frequency down indication and a frequencyup indication; and

a voltage-to-current converter circuit coupled to the sampling circuitand configured to convert the frequency control voltage to a frequencycontrol current in response to the switch circuit control signal, thevoltage-to-current converter circuit comprising an output switch circuitcontrolled by the switch control signal and configured to havesubstantially equal respective latencies for the frequency downindication and the frequency up indication; and

a filter coupled to the phase detector and the frequency detector andconfigured to output a control signal to the voltage controlledoscillator, wherein the control signal provides an indication of a phasedifference and a frequency difference between the input data signal andthe clock signal.

Clause 11. The system of clause 10, further comprising a re-timercircuit coupled to the voltage controlled oscillator and the phasedetector, the re-timer circuit configured to clock out the buffered datasignal in response to the clock signal.

Clause 12. The system of any one of clauses 10-11, wherein the switchcircuit control signal is coupled to one transistor, having a firsttransistor latency, to generate the frequency up indication and onetransistor, having a second transistor latency, to generate thefrequency down indication, wherein the first and second transistorlatencies are the same.

Clause 13. The system of any one of clauses 10-12, wherein the samplingcircuit comprises a first flip-flop, having a data input coupled to theclock signal and a clock input coupled to the delayed data signal, togenerate the switch circuit control signal.

Clause 14. The system of any one of clauses 10-13, wherein the samplingcircuit further comprises a second flip-flop coupled in series with athird flip-flop, the second flip-flop having a data input coupled to theclock signal and a clock input coupled to the input data signal, thethird flip-flop having a data input coupled to an output of the secondflip-flop and a clock input coupled to an output of the first flip-flop.

Clause 15. A method for generating a frequency control signal from areference-less frequency detector, the method comprising:

generating an up/down control voltage, wherein the up/down controlvoltage is indicative of a frequency difference between an input datarate and a clock frequency and comprises an up frequency indicationvoltage and a down frequency indication voltage:

generating a control signal based on the data rate and the clockfrequency; and

generating the frequency control current based on the up/down controlvoltage and the on/off control signal, wherein the on/off control signalcontrols an output switch circuit of a voltage-to-current convertercircuit such that a latency of the up frequency indication voltage isequal to a latency of the down frequency indication voltage through theoutput switch circuit.

Clause 16. The method of clause 15 wherein the control signal is anon/off control signal.

Clause 17. The method of any one of clauses 15-16, further comprising avoltage-to-current converter circuit operating in a frequency up modebased on the input data rate being greater than the clock signalfrequency.

Clause 18. The method of any one of clauses 15-17, further comprisingthe voltage-to-current converter circuit operating in a frequency downmode based on the input data rate being less than the clock signalfrequency.

Clause 19. The method of any one of clauses 15-18, further comprising:

the on/off control signal turning on a first transistor, having a firstlatency, in the output switch circuit to generate the frequency controlcurrent based on the up/down control voltage; and

the on/off control signal turning on a second transistor, having asecond latency, in the output switch circuit to generate the frequencycontrol current based on the up/down control voltage, wherein the firstlatency and the second latency are equal.

Clause 20. The method of any one of clauses 15-19, further comprisingincreasing the clock frequency of the voltage controlled oscillatorbased on the up frequency indication voltage and decreasing the clockfrequency of the voltage controlled oscillator based on the downfrequency indication voltage.

Clause 21. The method of any one of clauses 15-20, further comprisingthe frequency control current charging a loop filter to increase theclock frequency and the frequency control current discharging the loopfilter to decrease the clock frequency.

In the previous description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description of example embodiments is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

The functions or algorithms described herein may be implemented insoftware in one embodiment. The software may consist of computerexecutable instructions stored on computer readable media or computerreadable storage device such as one or more non-transitory memories orother type of hardware based storage devices, either local or networked.Further, such functions correspond to modules, which may be software,hardware, firmware or any combination thereof. Multiple functions may beperformed in one or more modules as desired, and the embodimentsdescribed are merely examples. The software may be executed on a digitalsignal processor, ASIC, microprocessor, or other type of processoroperating on a computer system, such as a personal computer, server orother computer system, turning such computer system into a specificallyprogrammed machine.

Although a few embodiments have been described in detail above, othermodifications are possible. For example, the logic flows depicted in thefigures do not require the particular order shown, or sequential order,to achieve desirable results. Other steps may be provided, or steps maybe eliminated, from the described flows, and other components may beadded to, or removed from, the described systems. Other embodiments maybe within the scope of the following claims.

1. A reference-less frequency detector circuit, comprising: a samplingcircuit configured to generate a frequency control voltage and a switchcircuit control signal based on a frequency difference between a clocksignal frequency and an input data rate, the frequency control voltagecomprising a frequency down indication and a frequency up indication;and a voltage-to-current converter circuit coupled to the samplingcircuit and configured to convert the frequency control voltage to afrequency control current based on the switch circuit control signal,the voltage-to-current converter circuit comprising an output switchcircuit controlled by the switch control signal and configured to havesubstantially equal respective latencies for the frequency downindication and the frequency up indication, wherein the output switchingcircuit comprises first and second transistors coupled in series witheach other and having a complementary type with each other, wherein anoutput node for the output switching circuit is a common node betweenthe first and second transistors.
 2. The circuit of claim 1, wherein theoutput switching circuit further comprises: a first series circuitcomprising the first and second transistors, a gate of the firsttransistor coupled to the output of the sampling circuit and a gate ofthe second transistor coupled to an inverse of the output of thesampling circuit; a second series circuit comprising third and fourthtransistors coupled in series with each other and having a complementarytype with each other, a gate of the third transistor coupled to theinverse of the output of the sampling circuit and a gate of the fourthtransistor coupled to the output of the sampling circuit, wherein thefirst and second series circuits are coupled together in parallel atfirst and second common nodes; a fifth transistor coupled between asupply node and the first common node; a sixth transistor coupledbetween a circuit reference node and the second common node; and avoltage divider network coupled between the supply node and the circuitreference node, the voltage divider network configured to provide avoltage to a common node between the third and fourth transistors. 3.The circuit of claim 2, wherein the circuit reference node is circuitground.
 4. The circuit of claim 2, wherein the first and thirdtransistors are p-type field effect transistors (PFETs) and the secondand fourth transistors are n-type field effect transistors (NFETs). 5.The circuit of claim 2, wherein the voltage-to-current converter circuitfurther comprises: a first pair of transistors coupled in series witheach other and having a complementary type with each other, wherein agate of one of the first pair of transistors is coupled to an output ofa sampling circuit and configured to operate in response to a firststate of the frequency control voltage; a second pair of transistorscoupled in series with each other and having a complementary type witheach other, wherein a gate of one of the second pair of transistors iscoupled to the output of the sampling circuit and configured to operatein response to a second state of the frequency control voltage, thesecond state being an inverse of the first state, wherein the first andsecond pair of transistors are coupled in parallel at a first commonnode and a second common node, the first common node coupled to thesupply node; and a current source coupled between the circuit referencenode and the second common node.
 6. The circuit of claim 5, whereinremaining transistors of the first and second pairs of transistors areeach coupled in a diode configuration and coupled to the supply node. 7.The circuit of claim 6, wherein a gate of the fifth transistor iscoupled to a gate of the remaining transistor of the second pair oftransistors.
 8. The circuit of claim 2, wherein the substantially equalrespective latencies for the frequency down indication and the frequencyup indication are each based on a one transistor latency.
 9. The circuitof claim 8, wherein the one transistor latency for the frequency upindication comprises the first transistor and the one transistor latencyfor the frequency down indication comprises the second transistor.
 10. Aclock/data recovery system comprising: a voltage controlled oscillatorconfigured to generate a clock signal; a phase detector coupled to thevoltage controlled oscillator and configured to generate a buffered datasignal and a phase difference control signal based on an input datasignal and the clock signal, wherein the phase difference control signalis indicative of a phase difference between the clock signal and theinput data signal; a reference-less frequency detector coupled to thevoltage controlled oscillator and configured to generate a frequencydifference control signal indicative of a frequency difference betweenthe input data signal and the clock signal, the frequency detectorcomprising: a sampling circuit configured to generate a frequencycontrol voltage and a switch circuit control signal based on thefrequency difference between the clock signal and the input data signal,the frequency control voltage comprising a frequency down indication anda frequency up indication; and a voltage-to-current converter circuitcoupled to the sampling circuit and configured to convert the frequencycontrol voltage to a frequency control current in response to the switchcircuit control signal, the voltage-to-current converter circuitcomprising an output switch circuit controlled by the switch controlsignal and configured to have substantially equal respective latenciesfor the frequency down indication and the frequency up indication; and afilter coupled to the phase detector and the frequency detector andconfigured to output a control signal to the voltage controlledoscillator, wherein the control signal provides an indication of a phasedifference and a frequency difference between the input data signal andthe clock signal.
 11. The system of claim 10, further comprising are-timer circuit coupled to the voltage controlled oscillator and thephase detector, the re-timer circuit configured to clock out thebuffered data signal in response to the clock signal.
 12. The system ofclaim 10, wherein the switch circuit control signal is coupled to onetransistor, having a first transistor latency, to generate the frequencyup indication and one transistor, having a second transistor latency, togenerate the frequency down indication, wherein the first and secondtransistor latencies are the same.
 13. The system of claim 10, whereinthe sampling circuit comprises a first flip-flop, having a data inputcoupled to the clock signal and a clock input coupled to the delayeddata signal, to generate the switch circuit control signal.
 14. Thesystem of claim 13, wherein the sampling circuit further comprises asecond flip-flop coupled in series with a third flip-flop, the secondflip-flop having a data input coupled to the clock signal and a clockinput coupled to the input data signal, the third flip-flop having adata input coupled to an output of the second flip-flop and a clockinput coupled to an output of the first flip-flop.
 15. A method forgenerating a frequency control signal from a reference-less frequencydetector, the method comprising: generating an up/down control voltage,wherein the up/down control voltage is indicative of a frequencydifference between an input data rate and a clock signal frequency, andcomprises an up frequency indication voltage or a down frequencyindication voltage; generating a control signal based on the data rateand the clock frequency; and generating the frequency control currentbased on the up/down control voltage and the on/off control signal,wherein the on/off control signal controls an output switch circuit of avoltage-to-current converter circuit such that a latency of the upfrequency indication voltage is equal to a latency of the down frequencyindication voltage through the output switch circuit.
 16. The method ofclaim 15, wherein the control signal is an on/off control signal. 17.The method of claim 15, further comprising: in response to detecting theinput data rate is greater than the clock signal frequency, configuringthe voltage-to-current converter circuit to operate in a frequency upmode.
 18. The method of claim 17, further comprising: in response todetecting the input data rate is less than the clock signal frequency,configuring the voltage-to-current converter circuit to operate in afrequency down mode.
 19. The method of claim 18, further comprising:turning on a first transistor in the output switch circuit using thecontrol signal, the first transistor having a first latency, to generatethe frequency control current based on the up/down control voltage; andturning on a second transistor in the output switch circuit using thecontrol signal, the second transistor having a second latency, togenerate the frequency control current based on the up/down controlvoltage, wherein the first latency and the second latency are equal. 20.The method of claim 15, further comprising: increasing the clockfrequency of the voltage controlled oscillator based on the up frequencyindication voltage, and decreasing the clock frequency of the voltagecontrolled oscillator based on the down frequency indication voltage.21. The method of claim 20, further comprising: performing one ofcharging a loop filter using the frequency control current to increasethe clock frequency, or discharging the loop filter using the frequencycontrol current to decrease the clock frequency.